This invention relates to systems that use magnetic fields to determine an object""s location and orientation.
As is known in the art, systems may use magnetic fields to determine the location and orientation of an object. Such systems use magnetic field measurements to indirectly determine the location and orientation of the object. These systems are useful, for example, in the medical field, because they are able to accurately locate an object in a patient""s body with only minimal intrusion into the body. The intrusion involves placing a small probe near the object to be located. Then, the 3-dimensional location and orientation of the probe is determined from the effect that the probe""s location and orientation have on magnetic field measurements.
The probe may be either a source or a sensor of a magnetic field. If the probe is a source, sensors exterior to the body measure the field produced by the probe. If the probe is a sensor, magnetic sources exterior to the body produce the fields being measured.
A common feature of systems that use magnetic field measurements to determine a probe""s location and orientation is that measured magnetic fields are nonlinear functions of the location and orientation. Due to the non-linearity, an iterative process is used to determine the probe""s location and orientation from the measured magnetic field values. The iterative process compares values of the magnetic field for guessed probe locations and orientations to the measured values. If the field values at a guessed location and orientation are close to the measured values, the guessed and actual probe locations and orientations are also close.
The iterative process uses a physical model for the probe""s environment. The physical model specifies the location and orientation of each field source. From the specified locations and orientations, laws of electrodynamics determine the field values.
In a first aspect, the invention features a process for detecting distortions to probe location or orientation determinations. The process includes measuring a plurality of magnetic field values, determining one of a probe""s location and a probe""s orientation from an extremum of an optimization function. The measured values depend on the probe""s location and orientation. The optimization function depends on the measured field values and a model for the measured field values. The process also includes indicating that a distortion to the act of determining exists in response to the extremum belonging to a preselected set.
In a second aspect, the invention features a system for magnetically locating a probe. The system includes a plurality of magnetic field sensors or sources, a probe, and a processor coupled to receive the measured values. The probe""s location and orientation affects measured magnetic field values. The processor detects either a system failure or a field distortion from a value of an optimization function. The optimization function depends on differences between the measured field values and field values predicted from a physical model.
In a third aspect, the invention features a calibration process. The process includes setting up a condition that distorts determinations of a probe""s location and orientation from measured field values, determining an actual probe location and orientation, and measuring magnetic field values that depend on the location and orientation. The process also includes finding an extremum of an optimization function and labeling the extremum as a value indicative of the condition. The optimization function depends on differences between the measured field values and field values calculated from a guessed probe location and position.
In a fourth aspect, the invention features a process. The process includes measuring a plurality of magnetic field values, estimating a location and orientation of a probe, and calculating magnetic field values for the estimated location and orientation. The measured field values are functions of the location and orientation of the probe. The process also includes determining a value of an error function that depends on differences between the measured and calculated field values and determining whether a distortion of the measured values has occurred from the value of the error function.
In a fifth aspect, the invention features a program storage device storing a computer executable program of instructions to perform one of the above-described processes.
Other features and advantages of the invention will be apparent from the description, drawings, and claims.